MYP Physics Phase Change Notes

Matter Changes State When Energy And Pressure Change

1. A phase or state change occurs when matter changes between the solid, liquid, and gas states.
2. During a phase change, the substance remains the same material, meaning no new substance is formed.
3. Only the arrangement, spacing, and motion of particles change.
4. Phase changes occur because particles gain or lose kinetic energy.

Energy and Particle Motion in Phase Changes

1. Phase changes occur because particles gain or lose kinetic energy.
2. When energy is added to a substance, particles move faster and may overcome attractive forces.
3. When energy is removed, particles slow down and attractive forces pull them closer together.
4. Whether particles can escape or remain close together determines the state of matter.

Evaporation (Liquid → Gas)

1. Evaporation is the process by which particles escape from the surface of a liquid and become a gas.
2. Evaporation occurs only at the surface of a liquid, not throughout the liquid.
3. It can happen at any temperature, as long as some particles have enough energy to escape.
4. Particles in a liquid are constantly moving and colliding with each other.
5. These collisions cause energy to be transferred unevenly between particles.
6. Some particles gain more kinetic energy than others.
7. The most energetic particles at the surface can overcome attractive forces and escape into the gas state.

Why Evaporation Cools A Liquid

1. The particles that escape are above-average energy particles.
2. When they leave, the particles left behind have a lower average kinetic energy, so the temperature of the remaining liquid decreases.
3. This explains everyday observations:
   1. A hot drink cools as water evaporates from its surface.
   2. Sweat cools your skin when it evaporates.
   3. Ocean water is cooled by evaporation from the surface, partly counteracting solar heating.

Evaporation Compared with Boiling

1. Evaporation occurs at the surface, while boiling occurs throughout the liquid.
2. Evaporation happens at any temperature, while boiling occurs at a fixed temperature.